1. Field of the Invention
The present invention relates to linear actuators, and more particularly to linear actuators that are capable of actuating a shaft in a downhole device between two or more stable, linear positions.
2. Background of the Related Art
Downhole operations conducted during the prospecting of hydrocarbons commonly require the need to move, switch, open or generally perform some type of low-force actuation. Such actuation is typically performed using solenoids that are selectively energized to accomplish the desired actuation. However, solenoids are only capable of energized movement in one direction. Consequently, solenoid movement is often opposed by a spring or another solenoid in order to move an actuator back to its original position following actuation by the first solenoid.
The use of an opposing spring is problematic in that the solenoid must compress the spring (i.e., overcome the spring force) to move the actuator from its original position, which requires more electrical energy than performing the actuation without the opposing spring force. Additionally, the solenoid must expend additional electrical energy to hold the actuator in the actuated position, until it is desired to permit the compressed spring to return the actuator to its original position. Actuators having a solenoid with an opposing spring are therefore inefficient from a power standpoint. Such inefficiency is amplified in a downhole application, because electrical power is expensive to generate and is often demanded by other downhole components as well.
The use of a second, opposing solenoid is also problematic in that the second solenoid requires its own coils and is typically configured in a collinear arrangement with the first solenoid. This results in an increased length and volume requirement within a downhole tool, which is expensive and often difficult to accommodate in a multi-functional tool.
The use of bistable actuators and linear motors for performing linear actuation is generally known, and such apparatuses have been employed to achieve linear actuation in downhole applications. Reference is made to the following U.S. Pat. Nos. 6,116,354 (“Rotary Steerable System for Use in Drilling Deviated Wells”); 5,531,270 (“Downhole Flow Control in Multiple Wells”); 5,236,047 (“Electrically Operated Well Completion Apparatus and Method”); 4,687,054 (“Linear Electric Motor for Downhole Use); as well as U.S. Patent Publication No. 2005/0109542 (“Steerable Drilling System”), each of which discloses the use of linear motors and/or bistable actuators for performing various linear actuation functions in downhole environments.
A shortcoming in existing linear motors and/or bistable actuators that are applied to downhole fluid flow control relates to the differential pressure across a flow control device such as a valve closure element. In particular, once a valve closure element is moved to a closed position, the differential pressure across the element may be relatively high, and tend to maintain the element in the closed position. This produces a situation where a relatively high linear actuation force is required to move the valve closure element from the closed position to an opened position. This need to overcome differential closing pressure therefore imposes a greater electrical energy demand on a downhole system than is needed, e.g., to simply perform linear actuation otherwise. The result is an inefficient consumption of electrical power.
A need therefore exists for an apparatus that achieves reliable linear actuation and operates under a minimum requirement of electrical power to move a valve closure element from its closed position. A need further exists for such an apparatus that is operable to achieve linear actuation between two or more stable linear positions.